Next Step: Proceed to 3-Hour 100g Oral Glucose Tolerance Test
A 1-hour glucose challenge test result of 159 mg/dL at 28 weeks gestation is abnormal and requires proceeding to a diagnostic 3-hour 100g oral glucose tolerance test (OGTT) to confirm or rule out gestational diabetes mellitus. 1, 2
Understanding the Two-Step Screening Approach
Your result falls within the standard two-step screening strategy used in the United States:
Step 1 (Completed): The 50g glucose load test (non-fasting) at 24-28 weeks showed 159 mg/dL at 1 hour.
- Threshold for proceeding: ≥130,135, or 140 mg/dL depending on local practice 1, 2
- Your result of 159 mg/dL exceeds all commonly used thresholds
- This screening test alone does NOT diagnose GDM—it identifies who needs diagnostic testing
Step 2 (Required Next): Perform a fasting 3-hour 100g OGTT 1, 2
The Diagnostic 100g OGTT Protocol
The test must be performed under specific conditions:
- Fasting state: Overnight fast of 8-14 hours 3
- Timing: Morning administration
- Preparation: At least 3 days of unrestricted diet (≥150g carbohydrate daily) and normal physical activity 3
- During test: Remain seated, no smoking 3
Diagnostic Criteria (Carpenter-Coustan)
GDM is diagnosed when at least 2 of the following 4 values are met or exceeded 1, 2:
- Fasting: ≥95 mg/dL (5.3 mmol/L)
- 1-hour: ≥180 mg/dL (10.0 mmol/L)
- 2-hour: ≥155 mg/dL (8.6 mmol/L)
- 3-hour: ≥140 mg/dL (7.8 mmol/L)
Important caveat: ACOG notes that some practitioners may diagnose GDM with only one elevated value, though the traditional criterion requires two 1, 2.
Why This Result Matters
Your 1-hour screening value of 159 mg/dL carries clinical significance:
- Research shows that screening values ≥140 mg/dL (versus <120 mg/dL) are associated with increased odds of adverse perinatal outcomes, large-for-gestational-age infants, and macrosomia (adjusted OR 1.48-1.97) 4
- Values in the 155-160 mg/dL range have been associated with fetal hyperinsulinism 5
- Even if the diagnostic OGTT is normal, this elevated screening result indicates higher baseline risk
Common Pitfalls to Avoid
- Do not skip the diagnostic OGTT: The screening test alone cannot diagnose or exclude GDM
- Do not use the 75g OGTT criteria: The two-step approach requires the 100g test with different thresholds 1
- Ensure proper test preparation: Inadequate carbohydrate intake or fasting duration can yield false results 3
- Do not delay: Schedule the diagnostic test promptly while still in the optimal 24-28 week window
If GDM is Diagnosed
Treatment begins with medical nutrition therapy and glucose monitoring, targeting 6:
- Fasting glucose <95 mg/dL (5.3 mmol/L)
- 1-hour postprandial <140 mg/dL (7.8 mmol/L) OR
- 2-hour postprandial <120 mg/dL (6.7 mmol/L)
Insulin therapy is added if lifestyle modifications fail to achieve these targets 3, 6.